1. Field of the Invention
The present invention relates to a method for communicating between base stations and mobiles to provide an accurate timing source between unsynchronized base stations and more particularly to a method for communicating between base stations and mobiles to provide an accurate timing source between unsynchronized base stations in a wireless TDMA or AMPS system.
2. Description of Related Art
The increased popularity of wireless telephony including cellular, enhanced specialized mobile radio (ESMR) and personal communication services (PCS), has resulted in a surge in wireless calls to 911. Public safety agencies strive to provide the same level of service to cellular callers as for land line callers, but they are hampered by significant technological diversity. Enhanced 911 systems were designed and constructed to support land lines telephones. Land lines telephones are connected via fixed wires back to Telco Central Offices (TCOs), and thus there is a permanent relationship between the telephone and its location. When someone places a 911 call from a land line telephone, the location of the phone is used to determine the closest police, fire, medical agency and public safety answering point (PSAP). Having this detailed operation at the operator""s fingertips enables the rapid response for which 911 systems are known.
However, wireless callers are not associated with any location, which is why they are so popular. This benefits works against a wireless telephone or mobile use that requires emergency assistance. When a caller""s geographic location is unknown, an operator must spend valuable time at the start of the call obtaining basic information about the location and area police, fire, medical agencies and PSAPs before fully understanding the nature of the emergency. Therefore, a quick determination of a caller""s location can be crucial for a person""s safety.
Wireless telephone base stations with mobile location systems are being considered to determine the geolocation of a mobile. Since the advent of radar, high accuracy location systems have used time difference of arrival (TDOA) methods as a preferred technology for location determination. In fact, the Global Positioning System (GPS) is a TDOA based system as are most of the systems proposed for the location and monitoring service being allocated by the FCC. TDOA systems operate by placing location receivers at multiple base stations geographically disbursed in a wide area. When a signal is transmitted from a mobile, the signal propagates at approximately a thousand feet per second to local base stations, where the signal reception is time stamped. The differences in time stamps are then compared using well known triangulation methods to produce intersecting hyperbolic lines from which the location is estimated.
However, TDOA methods require the base stations used in the triangulation methods to be accurately aligned in time. In other words, the base stations must be synchronized. Currently in TDMA and AMPS wireless systems, external equipment must be added to all of the individual base stations to provide the required timing accuracy and synchronization.
CDMA is a very different technology than TDMA or AMPS. CDMA has an N=1 frequency reuse plan where the same frequency spectrum is used by neighbor cells (to increase capacity). The synchronization of CDMA cells allows digitally coded voice data to be seamlessly handed off to a neighbor cell. TDMA has an N=7 frequency reuse plan and the individual 30 kHz frequency channels are not reused by adjacent cells but reused by cells far enough away to minimize co-channel interference.
Therefore, it is desirable to provide a method to provide timing synchronization between base stations in TDMA and AMPS systems without the implementation of expensive external equipment.
The present invention improves upon conventional TDMA and AMPS systems by providing an approach to use a TDMA/AMPS air interface to provide timing synchronization between unsynchronized base stations. This is accomplished in a preferred embodiment by marking time stamps of signals arriving at predetermined or candidate base stations from a serving base station. The time stamps are marked in relation to a clock signal of the respective candidate base station. The respective candidate base station becomes synchronized with the serving base station based on a down link offset calculated at each candidate base station that represents the fractional symbol time, relative to the internal symbol clock of the respective candidate base station, between the time the signal is transmitted from the serving base station to a next predetermined cyclical point of the symbol clock of the candidate base station. The predetermined cyclical point may be, for example, at the beginning of the next cycle when clock goes high. The respective candidate base station determines the time the signal was transmitted based on the time the signal is received at the candidate base station and the known time required for the signal to travel from the serving base station to the candidate base station.
Since the serving base station transmits the signal at a known point of its symbol clock cycle, for example the beginning of a clock cycle, the candidate base station can synchronize its internal symbol clock to the internal symbol clock of the serving base station. The synchronization is established by aligning the calculated down link symbol offset to correspond to the signal transmission time at the serving base station.
Although the internal symbol clocks of the base stations are not synchronized, they do operate at the same frequency based on the accurate and conventional T1 link clock. In other words, each clock operates at a frequency of about 1.544 MHz plus or minus 75 Hz. Accordingly, base station symbol clocks maintain approximately the same relationship in alignment to other base station symbol clocks. Therefore, two base stations that are in sync will remain synchronized for subsequent communication. When base stations are in sync, it is possible to perform operations that require synchronized stations, such as TDOA information used for determining the location of a mobile. Therefore, this invention provides a method for synchronizing base stations such that a mobile""s location can be determined using conventional TDOA methods.
Determining a mobile""s position provides a greater benefit than the application of emergency management from 911 calls. Other applications, such as fraud management, location sensitive billing, vehicle and fleet management, inventory/package monitoring, and wireless system design would also become available to TDMA and AMPS systems without severe hardware implementation costs.
In a preferred embodiment, the synchronization system is a software implementation on a computer used to determine the geographic location of the mobile. The software implementation receives a time stamp of receipt of a first signal transmitted from the serving base station to a candidate base station. The time stamp is measured in relation to the symbol clock of the candidate base station. The synchronization system determines the time the signal was transmitted from the serving base station, as a transmission time stamp, based on the receiving time stamp of the signal and the known distance between the serving base station and candidate base station. This transmission time stamp is measured in relation to the symbol clock of the candidate base station. The synchronization system then synchronizes the symbol clock of the candidate base station to the symbol clock of the serving base station by aligning the determined transmission time stamp of the signal to the known point of the serving base station symbol clock cycle that represents the actual transmission time stamp from the serving base station.